Traffic driven variable bandwidth optical transmission

ABSTRACT

Link bandwidth is varied based on the subscriber traffic load. Varying the link bandwidth has the effect of varying the actual noise margin of the link (in an inverse relation), so that the noise margin will vary inversely with the traffic load. A beneficial result is that, because the noise margin is increased during “off-peak” traffic periods, rapidly varying and burst impairments can be absorbed without causing data loss. In effect, the respective probability distributions of error bursts and traffic load are separated. Data loss only becomes a significant risk when peaks in both distributions coincide. However, the probability of that event occurring is comparatively low. This enables a lower noise margin allocation during design of the link, which dramatically reduces the link cost.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is the first application filed for the present invention.

MICROFICHE APPENDIX

Not Applicable.

TECHNICAL FIELD

The present invention relates to optical transmission systems, and inparticular to traffic driven variable bandwidth optical transmission inan optical communications network.

BACKGROUND OF THE INVENTION

It is well known that the bandwidth of an optical communication link islimited by the noise margin required to ensure reliable communication.Typically, the noise margin is measured in terms of a signal-to-noiseratio at the receiver end of a link. In some cases, the opticalsignal-to-noise ratio (OSNR) is directly measured at the receiver. Inother cases, signal parameters such as the eye opening, or bit errorrate (BER) detected at the receiver are used as a proxy for the signalto noise ratio. In all cases, the noise margin can be allocated to fourcategories of phenomena:

-   -   Constant impairments, such as insertion losses and polarization        coupling effects, which may vary between sites, or between        individual pieces of equipment, but do not change with time;    -   Slowly Varying impairments, such as temperature effects,        polarization phenomena in buried cable and aging, all of which        have autocorrelation times of greater than one second;    -   Rapidly varying impairments, which are short-term transients        having autocorrelation times of between one microsecond and one        second. Typical examples of rapidly varying impairments include        polarization mode dispersion due to above-ground cable movement,        and optical power transients; and    -   Bursts with autocorrelation times of less than one microsecond.

Typically, a noise margin of between 3 dB and 10 dB will be allocated toa link of the optical network, depending on the degree of reliabilityrequired and the specific unknown or varying parameters. This allocationis “static”, in the sense that it is selected based on the design of thelink and its involved network equipment. In general, the allocated noisemargin will be used in combination with forward error correction (FEC)to ensure that the link conveys subscriber traffic substantially withouterrors (e.g. BER≦10⁻¹⁵).

Within the optical network backbone, Synchronous Optical Network (SONET)Synchronous Transport Signalling (STS) and/or Synchronous Data Hierarchy(SDH) signalling is used extensively, because of its high bandwidthcapacity and reliability. Within such synchronous networks, symbols areconveyed through each link at a fixed rate, irrespective of the actualsubscriber traffic load at any instant. In order to maintain stabilityand synchronization across the network, any symbols that are notrequired for subscriber traffic (and control signalling) are encodedwith spectrally white pseudorandom data.

As is well known in the art, subscriber traffic is highly variable, withdaily, weekly and yearly patterns. For example, one-hour averages ofnight-time traffic may be only 10% that of mid-day traffic levels.During a given “peak busy hour”, the traffic load on large backbonelinks tends toward a Poisson distribution. As a result, theinstantaneous traffic load within a link of the optical network willfrequently be significantly lower than the (noise margin limited) linkbandwidth.

The difference between the actual traffic load at any instant and thelink bandwidth is frequently referred to as the link “headroom”. As thetraffic load increases, the headroom decreases with an attendant rise inthe risk of delayed or discarded packets, which is undesirable.Typically, for optical links in the network backbone, network operatorsprovide a headroom of about a factor of four between the average “peakbusy hour” traffic load and the link bandwidth. Clearly, theprovisioning of such large amounts of headroom is expensive, because itrequires that the network supplier lease significantly more linkbandwidth than will actually be used, on average.

A technique that reduces network costs remains highly desirable.

SUMMARY OF THE INVENTION

An aspect of the present invention provides a method of variablebandwidth optical transmission of subscriber traffic through a link ofan optical communications network. In accordance with the invention, atraffic load within the link is monitored, and the link bandwidthadjusted based on the traffic load.

A further aspect of the present invention provides a method ofcontrolling a noise margin of a link of an optical communicationsnetwork. In accordance with the invention, a traffic load within thelink is monitored, and the link bandwidth adjusted based on the trafficload.

Thus, for example, the link bandwidth can be reduced during periods oflow traffic load. This can be accomplished across the entire link or ona per-channel basis, if desired. In either case, reducing bandwidthincreases the actual (as opposed to the allocated) noise margin, andthereby improves system noise tolerance, particularly for rapidlyvarying and burst impairments. Because traffic loads are typically wellbelow the maximum bandwidth for most of the time, a lower average noisemargin can be allocated. This allows an increase in link length and/or areduction in the cost of network equipment required to obtain a desired(maximum) bandwidth. In addition, network providers can provision lessheadroom for a given expected peak traffic load, thereby furtherlowering costs.

Adjusting the link bandwidth can be accomplished by adjusting any one ormore of: a transmit optical power of a channel; a channel symbol rate; anumber of bits encoded within each symbol conveyed through a channel; aFEC encoding scheme used within a channel; a number of channelsmultiplexed within the link; and a channel spacing.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following detailed description, taken in combinationwith the appended drawings, in which:

FIGS. 1 a and 1 b are block diagrams schematically illustratingtransmitting and receiving nodes, respectively, of a conventionaloptical communications system;

FIGS. 2 a and 2 b are charts showing representative subscriber trafficload and signal impairments, respectively, as a function of time;

FIG. 3 is a block diagram schematically illustrating a transmitting nodein accordance with an embodiment of the present invention;

FIG. 4 is a block diagram schematically illustrating a receiving node inaccordance with an embodiment of the present invention; and

FIGS. 5 a and 5 b are representative subscriber traffic load and signalimpairment charts showing operation of the present invention;

FIG. 6 is a block diagram schematically illustrating a transmitting nodein accordance with a second embodiment of the present invention.

It will be noted that throughout the appended drawings, like featuresare identified by like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a method and apparatus for enablingtraffic driven variable bandwidth optical transmission in an opticalcommunications system. In order to facilitate understanding of thepresent invention, a brief description of prior art transmission systemsis provided below with reference to FIGS. 1 and 2. Embodiments of thepresent invention will then be described with reference to FIGS. 3-6.

FIG. 1 a is a block diagram schematically illustrating principaloperations of a transmitting node 2 of an optical communications system.As shown in FIG. 1 a, asynchronous subscriber signal traffic withinmultiple tributaries 4 is received by the node 2 and buffered in anelastic store 6. The traffic may comprise any arbitrary mix of signals,including Asynchronous Transfer Mode (ATM), Internet Protocol (IP), andGigabit Ethernet traffic. Traffic within each tributary 4 is normallybuffered in a respective First-In-First-Out (FIFO) buffer 8. Asynchronizing framer 10 reads data from each FIFO 8, and maps the readdata into corresponding tributaries of a number of SONET SynchronousPayload Envelopes (SPEs) 12, using a format such as the Generic FramingProtocol (GFP) standard. Each SPE 12 is then passed to a channeltransmitter (Tx) 14, which inserts the SPEs into an STS frame, and thenmodulates the STS frame onto an optical channel carrier 16 fortransmission through the optical link. A Tx local clock 18, which issynchronous with a SONET Primary Reference (not shown), is used tocontrol operation of the Synch. framer 10 and channel Tx 14.

As is known in the art, the number and size of the SPEs 12 are selectedbased on the channel line rate. For example, for an optical link havinga channel line rate of 10 Gb/s, the synchronizing framer 10 may mapsubscriber data into a set of four STS-48 envelopes. Other combinationsmay equally be used, such as, for example, eight STS-12 envelopes.

Normally, a respective buffer fill signal 20 is generated for eachtributary FIFO 8, and is used to control the insertion of idle packetsinto the corresponding SPE tributary. Thus, for example, if subscribertraffic is not present in any tributary 4 (as will be evidenced by anempty tributary FIFO 8), the synchronizing framer 10 will insert one ormore idle packets into the corresponding SPE tributary in place of the“missing” subscriber traffic. These idle packets are typically filledwith logical 1's or logical 0's, and are converted into pseudorandomdata fill during conventional encoding of the STS frame fortransmission.

In a Wavelength Division Multiplexed (WDM) optical communicationssystem, the above described process of receiving and bufferingsubscriber traffic; mapping buffered data to SPEs 12; inserting the SPEs12 into STS frames; and then modulating the STS frames onto an opticalcarrier 16 will be performed, in parallel, for each channel 22 of thesystem. The resulting parallel optical carrier (channel) signals 16 arethen optically multiplexed (at 24) into a WDM signal 26 that is launchedthrough the optical fiber link.

As shown in FIG. 1 b, at a receiving end of the link, the aboveprocessing steps are effectively mirrored to recover the originalsubscriber signal traffic. Thus, an incoming WDM optical signal 26 isdemultiplexed (at 28), and each channel signal 30 supplied to arespective channel receiver (Rx) 32. The channel Rx 32 detects anddecodes symbols within the channel signal 30 to recover the original STSframe.

As described above, the number and size of the SPEs 12, and the size ofSTS frames are selected based on the line rate, which, in turn, isderived from the Tx clock 18 (and thus the SONET primary reference). TheTx clock frequency is chosen to support the given line rate that can beaccommodated by the network equipment forming the link. For example, inthe network backbone, most network equipment is designed for a bandwidthof 10 Gb/s (line rate) in each channel 22.

As may be seen in FIG. 2 a, the subscriber traffic load typically varieswidely as a function of time. Typically, network service providers willprovision network links such that the available link bandwidth (BW), or,equivalently, the total SPE payload capacity, exceeds the expectedsubscriber traffic load. Idle packets provide pseudorandom data fill toreplace the “missing” data, in order to ensure that the SPEs (and thusthe STS frame) are full. This is required in order to avoid extended“gaps” in the channel carrier 16,30 which could have the effect ofdestabilizing the clock recovery circuit 40 at the receiving end of thelink.

Thus it will be appreciated that the conventional link equipmentoperates at its design line rate at all times, independently of theactual subscriber traffic load at any particular instant. As shown inFIG. 2 b, signal impairments also vary widely in time. However, becausethe line rate is a fixed value that is independent of subscriber trafficload, there will typically be no correlation between subscriber trafficload and signal impairments, as may be seen in FIGS. 2 a and 2 b.Because some signal impairments (such as self-phase modulation andcross-phase modulation) are proportional to the line rate, use of afixed line rate means that signal impairments are maintained at amaximum level. This, in turn, implies that the communications equipmentis continuously operating with a minimum noise margin consistent withsatisfactory transmission reliability (e.g. BER10⁻¹⁵). This leaveslittle room for error in the allocated noise margin, so networkdesigners must allocate a large enough amount of noise margin to allowfor unexpected events. The chart of FIG. 2 b shows three “error bursts”in which the signal noise in the link spikes above the maximum tolerablenoise value (I_(MAX)) defined by the noise margin, thereby resulting inlost or severely errored bits. It should be noted that, for a given linerate and FEC encoding scheme, I_(MAX) will be substantially constant, asillustrated in FIG. 2 b. As mentioned previously, the noise marginallocation and FEC encoding scheme are normally selected to provablyreduce the probability of such “lost bit” events to an acceptable value.While this results in a noise margin allocation that is appropriate forthe link bandwidth, such an allocation will normally be in excess ofthat which would be suggested by the actual subscriber traffic load atany particular instant.

The present invention provides a method of varying the link bandwidthbased on the subscriber traffic load. Varying the link bandwidth has theeffect of varying the actual noise margin of the link (in an inverserelation), so that the noise margin will vary inversely with the trafficload. A beneficial result of the present invention is that, because thenoise margin is increased during “off-peak” traffic periods, rapidlyvarying and burst impairments can be absorbed without causing data loss.In effect, the present invention separates the respective probabilitydistributions of error bursts and traffic load. Data loss only becomes asignificant risk when peaks in both distributions coincide. However, theprobability of that event occurring is comparatively low. This enables alower noise margin allocation during design of the link, whichdramatically reduces the link cost. Embodiments of the present inventionare described below with reference to FIGS. 3-6.

FIG. 3 is a block diagram illustrating a transmitting node 2 a inaccordance with an embodiment of the present invention. As shown in FIG.3, the present invention provides a variable channel transmitter (Tx) 46controlled by a Tx controller 48. As with the conventional channel Tx′14 of FIG. 1 a, the variable channel Tx 46 operates to map SPEs 12 fromthe synch. framer 10 into an STS frame, and then modulate the STS frameonto a respective channel optical carrier signal 16. However, inaccordance with the present invention, this functionality can becontrolled to vary the bandwidth of the channel optical carrier signal16.

In particular, the variable channel Tx 46 is designed to transmit anoptical carrier signal 16 having a desired bandwidth. In the embodimentillustrated in FIG. 5 a, a desired one of four different bandwidths(BW1-BW4) can be selected. These bandwidths, may, for example,correspond to OC-48, OC-96, OC-144 and OC-192 bandwidths. Variousmethods may be used to select an appropriate bandwidth based on thesubscriber traffic load. For example, a set of threshold values(Th1-Th3) can be determined based on the objective of maintaining apredetermined minimum amount of headroom. The buffer fill 20 can then becompared to the threshold values, and the appropriate bandwidth selectedbased on the comparison result.

As shown in FIG. 5 b, changing the bandwidth produces correspondingchanges in the instantaneous noise margin, which is reflected in themaximum tolerable noise in the link (L_(MAX)). In particular, I_(MAX)varies inversely with the line rate, as may be seen in FIG. 5 b. Theminimum value of I_(MAX) is obtained only at the maximum line rate(BW4). Consequently, the risk of a “lost bit” event is highest only whena spike in the link noise coincides with a peak in the line rate (andthus the subscriber traffic load). Because the link noise and subscribertraffic distributions are largely uncorrelated, the probability ofcoincident link noise spikes and traffic peaks is very low. Thus,varying the bandwidth in accordance with the present inventiondramatically reduces the probability of “lost bit” events.

A further advantage of the present invention is that changes in thebandwidth are effected at the transmitting node 2 a, based on thedetected subscriber traffic load. Because the traffic load detection andbandwidth control functions are performed locally at the transmittingnode 2 a, speed of light propagation delays (about 1 mSec per 1000 km offiber per direction) do not impose any significant limitations on theresponse time of the control function. Thus the present invention iscapable of responding to even very short duration changes in thesubscriber traffic load, and tolerate fast link changes without needingto respond.

Various methods may be used to vary the channel bandwidth, including:

-   -   varying a line rate of the channel;    -   varying a number of bits encoded within each symbol conveyed        through the channel; and    -   varying a FEC encoding scheme of the channel.        Each of these techniques is described in greater detail below.        In each case, the buffer fill can conveniently be used as an        indicator of the client traffic load.        Varying a Line Rate of the Channel

In this method, the rate at which symbols are modulated onto the channeloptical carrier 16 is controlled. Varying the line rate directlycontrols the noise margin of the involved channel, because lower linerate signals are inherently more noise tolerant. In addition, varyingthe line rate on one channel indirectly alters noise margin in adjacentchannels, by changing cross-channel effects such as cross-phasemodulation, which are line-rate sensitive.

In general, varying the line rate involves selecting a desired Tx clockfrequency, and selecting an appropriate STS frame size. In principle,this can be accomplished to a minimum granularity of an SPE-1 (i.e. 52Mb/s). However, in practice, it may be preferable to use a larger stepsize. For example, in the embodiment of FIG. 3, the synch. framer 10maps data from the tributary FIFOS 8 to a set of four SPEs 12. In orderto provide a maximum bandwidth of 10 Gb/s, for example, each of the fourSPEs 12 will be a concatenated STS-48 envelope. With this arrangement, amaximum of four different line rates can be accommodated, correspondingto OC-48, OC-96, OC-144 and OC-192 bandwidths, respectively. The Txclock circuit can readily be designed to generate the required clockfrequencies of 2.5 GHz, 5 GHz, 7.5 GHz and 10 GHz, respectively, inorder to support each of these bandwidths. A selector switch 50controlled by the Tx controller 48 can then be used to select theappropriate clock frequency, so as to drive the synch. framer 10 and thevariable channel Tx 46 at the desired frequency.

As shown in FIG. 4, a similar arrangement is provided at the receivingend of the link. Thus, the clock recovery circuit 40 is constructed togenerate recovered clock signals having the required frequencies (inthis case 2.5 GHz, 5 GHz, 7.5 GHz and 10 GHz). An Rx controller 52 cancontrol a selector switch 54 to select the appropriate recovered clocksignal, so as to drive the pointer processor 34 at the line rate of theincoming optical carrier signal 30. Because all of the recovered clocksignals 38 are phase locked with the received channel optical carriersignal 30, the Rx controller 52 can switch between different Rx clockfrequencies, without data loss due to loss of phase lock.

With this arrangement, the variable channel Tx 46 can transmit anoptical carrier signal 16 having a desired one of four different linerates. Various methods may be used to select an appropriate line ratebased on the traffic load. For example, a set of threshold values can bedetermined based on the objective of maintaining a predetermined minimumamount of headroom. The buffer fill 20 can then be compared to thethreshold values, and the appropriate channel line rate selected basedon the comparison result.

In order to prevent data loss due to mismatch between the Tx and Rxclock frequencies, the variable channel Tx 46 can be controlled toinsert information identifying the line rate (or a line rate change)into the channel signal 16, for example using one or more undefinedbytes of the STS frame overhead. For example, if it is desired to changethe line rate, then the new line rate can be inserted into the overheadof the STS frame immediately preceding the line rate change (that is,the last frame transmitted at the old line rate). When the STS framearrives at the receiving end of the link, the line rate (or line ratechange) is detected by the pointer processor 34, and passed to the Rxcontroller 52 (at 56). The Rx controller 52 can then select theappropriate recovered clock signal 38 for the next successive frame. Bythis means, the line rate can be varied between successive STS frameswithout data loss.

In the illustrated embodiment, four different line rates may beselected. As will be appreciated, a larger or smaller number ofdifferent line rates may be used.

Varying a Number of Bits Encoded within Each Symbol Conveyed Through theChannel

As is known in the art, multi-level signals can be used to encode two ormore bits within each symbol. For example, In U.S. Pat. No. 5,408,498,which issued to Yoshida on Apr. 18, 1995, a quaternary (i.e. four-level)signal is used to encode two bits in each symbol. In principle, thispermits the channel bandwidth to be increased (in this case by a factorof two) without altering the line rate. However, it also reduces thenoise margin, because accurate recovery of subscriber traffic by thechannel Rx 32 requires more accurate detection of the analog level ofthe incoming channel optical carrier 30.

In this method, the number of bits encoded into each symbol iscontrolled, to thereby directly control the noise margin of the involvedchannel. Various methods may be used to select an appropriate number ofbits-per-symbol based on the traffic load. For example, a set ofthreshold values can be determined based on the objective of maintaininga predetermined minimum amount of headroom. The buffer fill can then becompared to the threshold values, and the appropriate number ofbits-per-symbol selected based on the comparison result.

At the receiving end of the link, correct detection and decoding ofsubscriber traffic requires that the channel Rx 32 be controlled todetect 2^(N) (where N is the number of bits encoded within each symbol)discrete analog levels of the inbound channel optical carrier 30. Thismay be accomplished by controlling the variable channel Tx 46 to insertthe number (N) of encoded bits into the channel signal 16, for exampleusing one or more undefined bytes of the STS frame overhead. Forexample, if it is desired to change the number of encoded bits, then thenew number can be inserted into the overhead of the STS frameimmediately preceding the change (that is, the last frame transmittedusing the old number of encoded bits). When the STS frame arrives at thereceiving end of the link, the number of encoded bits within each symbol(or the change in that number) is detected by the pointer processor 34,and passed to the Rx controller 52. The Rx controller 52 can thencontrol the channel Rx 32 (at 58) to apply the appropriate slicinglevels for detection of the next successive frame. By this means, thenumber of bits encoded within each symbol can be varied betweensuccessive STS frames without data loss.

Varying a FEC Encoding Scheme of the Channel

As is known in the art, it is common to allocate approximately 7% of alltransmitted bits related to Forward Error Correction (FEC symptoms). Ifstronger FEC encoding is used, the additional bits required for FECsymptoms inherently reduces channel bandwidth (in terms of the capacityto carry subscriber traffic), but noise tolerance (and thus noisemargin) increases, without any other change in the system.

In this method, the FEC encoding scheme is controlled to therebydirectly control the noise margin of the involved channel. Variousmethods may be used to select an appropriate FEC encoding scheme basedon the traffic load. For example, a set of threshold values can bedetermined based on the objective of maintaining a predetermined minimumamount of headroom. The buffer fill 20 can then be compared to thethreshold values, and the appropriate FEC encoding scheme selected basedon the comparison result.

At the receiving end of the link, correct detection and decoding ofclient data traffic requires that the channel Rx 32 be controlled toapply the appropriate FEC decoding scheme to the inbound channel opticalcarrier. This may be accomplished by controlling the variable channel Tx46 to insert information identifying the desired FEC encoding schemeinto one or more undefined bytes of the STS frame overhead. For example,if it is desired to change the FEC encoding scheme, then informationidentifying the new scheme can be inserted into the overhead of the STSframe immediately preceding the change (that is, the last frametransmitted using the old FEC encoding scheme). When the STS framearrives at the receiving end of the link, the FEC scheme identifier isextracted by the pointer processor 34, and passed to the Rx controller.The Rx controller can then control the channel Rx 32 to apply theappropriate FEC encoding scheme to the next successive frame. By thismeans, the FEC encoding scheme can be varied between successive STSframes without data loss.

The embodiments and methods described above are designed to control thenoise margin of a channel by varying the bandwidth of that same channel.Clearly, these techniques can be independently implemented, alone or incombination, for each channel of a WDM optical communications system. Asis well known in the art, WDM system suffer impairments due tocross-channel effects. Consequently, a change in the bandwidth of onechannel will affect the noise margin in an adjacent channel. Thisphenomenon was described above as one of the effects of changing theline rate of a channel. However, additional bandwidth control methodsmay be implemented to exploit cross-channel effects, and thereby adjustnoise margin in adjacent channels. These methods are described belowwith reference to FIG. 6.

FIG. 6 is a block diagram schematically illustrating an embodiment ofthe invention in which bandwidth control can be implemented acrossmultiple channels of a WDM communications system. As shown in FIG. 6,each channel 22 is provided with an independent buffer 6, synch. framer10 and variable channel Tx 46, which operates substantially as describedabove with respect to FIG. 3. Thus, each channel 22 includes a buffer 6for receiving subscriber traffic through a respective set of tributaries4 from a conventional network router 60. The synch. framer 10 reads thebuffered data and maps the read data into a set of SPEs 12. The variablechannel Tx 46 inserts the SPEs into STS frames and modulates the framesonto a respective channel optical carrier 16.

Operation of each channel 22 is controlled by a shared Tx controllerunit 48, which receives a respective buffer fill signal 20 for eachchannel 22, and determines (and controls) the bandwidth of each channel22.

In conventional WDM systems, a shared Tx clock 18 is used to drive allof the channels, at least across a single shelf of the node. In theembodiment of FIG. 6, the Tx Clock 18 a is designed to generate multipleclock signals 62, as described above with reference to FIG. 3. A switcharray 64 enables each clock signal 62 to be supplied to any desiredchannel synch. framer 10 and Tx 46. By this means, the line rate of eachchannel 22 can be individually controlled.

As may be appreciated, the embodiment of FIG. 6 is capable ofimplementing all of the methods described above with reference to FIGS.3-5. Additional “cross-channel” methods which can be implemented usingthe embodiment of FIG. 6 include:

-   -   varying a channel optical power level;    -   varying a number of channels; and    -   varying a channel spacing.        Each of these techniques is described in greater detail below.        As in the “single channel” embodiments described above with        reference to FIGS. 3-5, the buffer fill can conveniently be used        as an indicator of the client traffic load.        Varying a Channel Optical Power Level

In this method, the optical power level of each channel is controlled,based on either the traffic load within the respective channel, or thecurrent bandwidth of that channel. As will be appreciated, reducing theoptical power within a channel will tend to degrade the noise margin ofthat channel. However, this can be offset by noise margin improvementsobtained by bandwidth reductions implemented in accordance with any ofthe other methods described herein. Thus as the traffic load (or channelbandwidth) is reduced, the transmit optical power level can also bereduced.

A primary advantage of varying the optical power level of each channelis that cross-phase modulation, four-wave mixing and Raman scatteringeffects are all proportional to the optical power level. Thus areduction in the optical power level of any channel reduces impairmentssuffered by adjacent channels within the link.

As may be appreciated, advantages of optical power control may belimited by the presence of certain optical equipment within a link. Forexample, optical channel equalizers are sometimes used to minimizeoptical power variations between adjacent channels. Clearly, such anoperation would tend to destroy the benefits of implementing individualper-channel power control. However, where client traffic is evenlydistributed across all of the channels, similar bandwidth reductions arelikely to be implemented within each channel. In this case, the powerlevel of all of the channels may be controlled together, in accordancewith the total traffic load through the link. With this arrangement, thepresence of optical channel equalizers within the link will notadversely impact the benefits of this method.

Varying a Number of Channels

In this method, the number of channels 22 multiplexed into the link isvaried. As may be appreciated, disabling a channel 22 has the effect ofeliminating impairments suffered by adjacent channels, due tocross-channel effects within the link. This approach can be consideredto be an extreme example of per-channel power reduction, in which theoptical power of one or more channels is reduced to zero. In order toprevent loss of data, the controller can notify the router 60 to directclient traffic to only those channels that are currently active.

Varying a Channel Spacing

As described above, varying the number of channels multiplexed withinthe link increases the noise margin of the remaining (active) channels,by eliminating cross channel impairments from the disabled channel(s).Further improvements in the noise margin of the active channels can beimproved by suitable selection of the channels to be disabled. Inparticular, by disabling alternate channels, the spacing between theactive channels will be increased. This increased channel spacingreduces cross-channel effects between the active channels.

The present invention provides methods and apparatus for controllinglink bandwidth based on the traffic load. The link bandwidth iscontrolled in response to changes in the buffer fill 20, and bandwidthchanges can be implemented on a per STS frame basis. Thus the bandwidthcan be updated every 125 μSec.

In the above description, link (or channel) bandwidth is controlledbased on traffic load. If desired, an additional level of bandwidthcontrol can be implemented, based on measured noise margin at thereceiving end of the link. Thus, for example, known methods can beimplemented at the Channel Rx 32 to measure the noise margin. The noisemargin may be measured directly, or a signal quality parameter such asOptical Signal to Noise Ratio OSNR, eye opening, or bit error rate maybe used as a proxy. In either case, the measured noise margin is thentransmitted to the Tx controller 48, as shown at 66 in FIG. 6. Uponreceipt of the noise margin measurement, the Tx controller 48 cancalculate a maximum permissible link bandwidth. This value can then beused as an upper limit for the traffic-driven bandwidth.

For example, during a slowly varying event such as a temperature effect,the Tx controller 48 will use the corresponding degraded noise marginmeasurements to calculate the maximum permissible link bandwidth.Referring back to FIG. 5 a, the calculated maximum permissible linkbandwidth may be lower than the highest “design” bandwidth BW4. Trafficload within the link may drive the actual (operating) link bandwidthbelow this level (i.e. to any of BW1-BW3), but it can not drive thebandwidth higher (i.e. to BW4).

Because this technique requires that the noise margin measurement betransmitted from the receiving node back to the transmitting node, thespeed of the control loop is subject to speed-of-light propagationdelays. As a result, the responsiveness of this level of bandwidthcontrol will be limited by the link length. For most links in theoptical network backbone, the response time will normally be fast enoughto track slowly varying impairments. The probability of rapidly varyingand burst impairments coinciding with peak traffic loads is expected tobe comparatively low. Accordingly, this technique allows a furtherreduction in the noise margin allocation used for calculating linkheadroom. This, in turn further reduces network costs, by allowingnetwork providers to provision less bandwidth capacity for a givenamount of subscriber traffic.

The embodiment(s) of the invention described above is(are) intended tobe exemplary only. The scope of the invention is therefore intended tobe limited solely by the scope of the appended claims.

1. A method of variable bandwidth optical transmission of asynchronoussubscriber traffic through a link of a synchronous opticalcommunications network, the method comprising, at a variable channeltransmitter transmitting a synchronous optical signal through an opticalchannel of the link to a receiver of the synchronous opticalcommunications network, steps of: monitoring a channel traffic load ofthe asynchronous subscriber traffic; selecting a new bandwidth of theoptical channel, in response to the monitored channel traffic load;transmitting information indicative of the new bandwidth through thelink to the receiver, and thereafter controlling the variable channeltransmitter to transmit the synchronous optical signal through theoptical channel in accordance with the selected new bandwidth.
 2. Amethod as claimed in claim 1, wherein the step of monitoring the channeltraffic load comprises a step of monitoring a fill of a respectivechannel buffer of the transmitter.
 3. A method as claimed in claim 2,wherein the step of selecting the new bandwidth of the optical channelcomprises steps of: comparing the monitored buffer fill to apredetermined threshold value; and selecting a desired channel bandwidthbased on the comparison result.
 4. A method as claimed in claim 3,wherein the step of selecting the desired channel bandwidth comprisesany one or more of: varying a transmit optical power of the channel; andtemporarily disabling the channel.
 5. A method as claimed in claim 3,wherein the step of selecting the desired channel bandwidth comprisesselecting a desired line rate of the optical channel.
 6. A method asclaimed in claim 5, wherein the step of controlling the variable channeltransmitter comprises: selecting a synchronous payload envelope size inaccordance with the desired line rate; selecting a transmit clock of thetransmitter in accordance with the selected synchronous payload envelopesize; and controlling the variable channel transmitter to transmit thesynchronous optical signal through the optical channel using theselected synchronous payload envelope size and the selected transmitclock.
 7. A method as claimed in claim 6, wherein the informationindicative of the new bandwidth comprises an indication of the selectedline rate.
 8. A method as claimed in claim 3, wherein the step ofselecting the desired channel bandwidth comprises selecting a desirednumber of bits encoded within each symbol conveyed through the opticalchannel.
 9. A method as claimed in claim 8, wherein the informationindicative of the new bandwidth comprises an indication of the selectednumber of bits encoded within each symbol.
 10. A method as claimed inclaim 8, wherein the step of controlling the variable channeltransmitter comprises controlling the variable channel transmitter to:encode the synchronous optical signal into symbols having the desirednumber of bits; and modulate the symbols onto an optical channel carrierfor transmission through the link.
 11. A method as claimed in claim 3,wherein the step of selecting the desired channel bandwidth comprisesselecting a desired FEC encoding scheme of the channel.
 12. A method asclaimed in claim 11, wherein the information indicative of the newbandwidth comprises an indication of the selected number of bits encodedwithin each symbol.
 13. A method as claimed in claim 11, wherein thestep of controlling the variable channel transmitter comprisescontrolling a FEC encoder of the variable channel transmitter to encodethe synchronous optical channel signal using the selected FEC encodingscheme.
 14. A method as claimed in claim 1, wherein the step oftransmitting information indicative of the new bandwidth to the receivercomprises inserting the information into an overhead of the synchronousoptical signal.
 15. A method as claimed in claim 1 further comprising,at a receiver receiving the synchronous optical signal through theoptical channel of the link from the variable channel transmitter, stepsof: receiving information indicative of a new bandwidth of the opticalchannel from the variable channel transmitter; and controlling thereceiver to thereafter receive the synchronous optical signal inaccordance with the new bandwidth.
 16. A method as claimed in claim 15,wherein the step of receiving the information indicative of the newbandwidth from the variable channel transmitter comprises extracting theinformation from an overhead of the synchronous optical signal receivedthrough the link.
 17. A method as claimed in claim 15, wherein theinformation indicative of the new bandwidth comprises an indication of aline rate, and wherein the step of controlling the receiver comprisesselecting a receive clock of the receiver in accordance with theindicated line rate for reception of a subsequent frame of thesynchronous optical signal.
 18. A method as claimed in claim 15, whereinthe information indicative of the new bandwidth comprises an indicationof a FEC encoding scheme, and wherein the step of controlling thereceiver comprises selecting a FEC decoding scheme in accordance withthe indicated FEC encoding scheme for decoding a subsequent frame of thesynchronous optical signal.
 19. A method as claimed in claim 15, whereinthe information indicative of the new bandwidth comprises an indicationof a number of bits encoded within each symbol, and wherein the step ofcontrolling the receiver comprises controlling a channel receiver toapply slicing levels for detection of a subsequent frame of thesynchronous optical signal, in accordance with the selected number ofbits encoded within each symbol.